Antenna pattern recorder



March 17, 7 1 9 59 M.'L. KUDER ANTENNA PATTERN RECORDER Original Filed March 18, 1946 2 Sheets-Sheet 1 VARIABLE RECEIVER ATTENUATOR 'll' J fl/Il \0 .E :6a I, PUSH-PULL I PEAK PULSE SERVO BALANCE VOLTAGE ACTUATOR DETECTOR .yiDETEUOR STRETCHER 5:

' v 8Y5? I v I REFERENCE I POTENTIAL wsouficE e 1.15:5. E PULSE- A STRETCHER PEAK oETEc RECEIVER Br PULSE RMS 1 ATTENUATOR 26 AMPLIFIER TOR 72 I. T

l TO BALANCE DETECTOR AND ASSOCIATED CIRCUITS 1|. 1 3 INVENTOR MILTON' L. KUDER ATTORNEYS ,United Stat s atentOfice 2,878,468 ANTENNA PATTERN RECORDER Milton L. Kuder, Washington, D. C.

application March 18, 1946, Serial No. 655,360, now Patent No. 2,763,859, dated September 18, 1956.

- and this application March 7, 1955, Serial No.

and used by or for the Government of the United States of 'America for governmental-purposes without the payrnent of any royaltiesthereon or therefor.

This invention'relates to the measurement and recording of the strength of electrical signals or of electromagnetic'fields and, more particularly, to the determination of the radiation pattern of an antenna system.

This application is a division of my co-pending application, Serial No." 655,360, filed March 18, 1946, now U. S. Patent No. 2,763,859, issued Sept. 18, 1956.

' Previous methods of determining radiation patterns have in general been laborious and slow, and have often introduced considerable error. This is conspicuously true in the measurement of the radiation patterns of antennas installed in aircraft. This invention eliminates errors caused by such things as fading that occur during the long time usually required in performing measurement. It is independent of the calibration of the receiver used in such measurements and also of the receivers dynamic range.

An object of thisinvention is to provide automatic,

rapid, and continuous measurement and simultaneous recording of'the radiation pattern of an antenna system. 'Another'objec't of this invention is to provide a suitable method of and means for automatically obtaining and recording the radiation pattern of an antenna installed in j aircraft.

Anothe'r object of this invention is to provide means for obtaining the'radiation pattern of an antenna by use of the radiated signal provided by a transmitting source supplying eitl 'er"pulse' type transmission, or modulated or unmodulated' continuous wave transmission.

Another object 'of this invention is to provide means tor obtaining the radiation pattern of an antenna by use "of any communications or pulse type receiver. 7

Another obiect of this invention is to provide means for obtaining thera'diation pattern of an antenna in a manner that isindependent of the characteristics of the receiver employed, provided the receiver is capable of detecting the weakest isig'nal'desired to be measured within the dynamicrange provided by such means. I

' A further object of this invention is to provide means for measuring a signal over a large range of instantaneous signal strength, such range being limited to no arbitrary value, in general being much greater than that of radio receivers in general use.

Other objects and features of the present invention will become apparent upon careful consideration of the following detailed description when taken together with the accompanying drawings. 1 Fig. 1. is. a schematic .diagraimpartlyin block, showing one embodiment of the invention;

Fig. shows, in part, a modified arrangement of the embodiment of Fig. 1;

j Fig. 3 is a'schematic diagram showing in greater detail a portion'of the apparatus illustrated in Fig. 1, and

Fig. 4'is a diagram illustrating the method of obtaining theradiationpattern ot'an'a'irbome antenna.

Patented Mar.

Briefly, and according to the invention, it is desired provide means to cause the signal received from the antenna under test to move an indicator, such as a sliding pointer, back and forth according to the strength of the signal. It is also desired to have this action occur'over a sutficient range of values and have the responseg'ive a true indication of the strength of the signal. .The an tenna to be tested is mounted in an airplane in the mag ner in which it is to be used. The signal from the antenna is coupled to the input of a variable high-frequency attenuator, the output of which is coupled to a receiver. The receiver amplifies and detects the signaland passes it to a peaking device which produces a steady D. C. output voltage that is proportional to the peak value ofithe output signal from the receiver. ThisjD.-C." voltage, i s compared to a fixed D.-C. voltage and the combination fed to an actuating mechanism which is geared'to the high-frequency attenuator and is able to move the attenuator in such a manner as to'give increased or decreased attenuation of the signal at the input of the receiver. If the two D.-C. voltages are equal the actuating mechanism does not move and the attenuator remains stationary; If the signal from the antenna becomes less, the receiver output and D.-C. voltage from the peaking device become less and the two D.-C. voltages are no longer equal. The actuating mechanism is energized to move the attenuator so that it gives less attenuation. This action continues until the input to the receiver is thereby increased to a point where the two D.-C. voltages are again equal. If the signal from the antenna should increase, the chain of operations will repeat but in opposite directions so that the attenuation is increased till the two D.-C. voltages are again equal. It is seen that the input to the receiver is kept constant'by this action so that the characteristics of the receiver do not appear in the result and thatthe range of instantaneous signal strength obtained is 'rneasti'red in terms of the attenuation required. z

What is now required is to gear an indicator and stylus to the moving part of the attenuator so that its actionl'is indicated and recorded. A record may be easily obtained by placing under the stylus a disk bearing a piece of polar graph paper, the disk being fixed in space in a horizontal plane by means of a gyroscope, and, that part of the apparatus including the stylus, attenuator, and associated equipment is mounted in the airplane, so'that it will move as the airplane turns in azimuth, thusproviding relative motion between stylus and paper that'is is assumed to be receiving a pulse modulated transmission. The signal from antenna 1 is coupled to variable attenuator 2 which in turn is electrically connected to receiver 3 which has volume control 4. The signal after being amplified by receiver 3 passes through variable input 5 to a peaking device 6 which comprises a pulse stretcher circuit 6b and peak voltage detector 6a.v The pulse stretcher 6b and peak voltage detector 6a widen the pulses until a steady direct current voltage propertional to the peak value of the pulse detected by, ',th,e receiver is obtained from any pulse transn'iission having a pulse repetition rate commonly encountered in such transmissions. The output of the peaking device 6 is fed in series with a voltage from direct current generator 12 to one input of the balanced detector 7 which compares the output voltagewith a fixed reference potential from source 8. The latter is adjustable'as indicated at 9 to accommodate the nominal mid-range output of the receiver 3. The output of balanced-detector 7 is fed into an actuating mechanism which in this embodiment is preferred to be a push-pull servo actuator. which, in turn, produces mechanical rotation of shaft 11 geared by means of, the rack and pinion 50 to the input section Ofattenuator 2. The character of this rotation is determinedby the output of the balanced-detector 7. If the directcurrent output voltage of peaking device 6 is identical 'to the reference potential supplied by reference source 8, the balanced-detector 7 produces a balanced output and shaft 11 of the servo actuator 10 remains stationary." If,however, the two voltages being compared arenot equal, the balanced-detector output is said to be unbalanced and shaft 11 will rotate either clockwise or counter-clockwise depending on the polarity of'the unbalance.

The direction of rotation of shaft 11 is such as to vary attenuator 2 in a way that will maintain the voltage constant at the input to receiver 3 and consequently restore the balance between the output of the peaking device 6 and reference potential 8.

Direct current generator 12 produces a small voltage that is proportional to the speed of shaft 11 and opposite in polarity to the unbalanced voltage between peaking device 6 and reference source 8. The efiect of this, together with the normal characteristic of servo actuator 10, produces a speed of rotation of shaft 11 that is proportional to the degree of unbalance. This means that there is virtually no lag in following sudden changes and that hunting is eliminated.

Since the receiver 3 input is maintained constant, the range of signal strength to be accommodated by the receiver3 need not be great nor must it have a linear response characteristic. Hence, almost any available receiver can be used with the system.

Attenuator 2 slides back and forth according to the strength of the signal obtained from antenna 1 so as to maintain a constant input to receiver 3 at the output of attenuator 2. The couplings of the attenuator will be widely separated when there is a very strong signal on the antenna, close together for a weak signal, and at an intermediate position for a moderate signal. Thus, the position of the attenuator may be considered a direct function of the antenna field pattern. In this embodiment the attenuator is so constructed (as hereinafter described) that'it offers an attenuation that varies logarithmically with displacement, so the displacement can be considered an indication of a change in field strength, measured in decibels. To obtain a polar plot 25 of field strength in a horizontal plane XY, it is only necessary to attach a stylus 13 to the movable part ofthe attenuator by means of a lever system 14 and place under it a piece of polar graph paper 15 mounted on a platform 16 stabilized in azimuth so as to remain fixed with respect to the earth as the antenna 1, stylus 13, attenuator 2, and associated equipment are turned about a vertical axis Z. If the equipment is mounted in a mobile installation such as an airplane, for which this embodiment is designed, this relative motion is accomplished by means of a gyroscope 17 which, when set to spinning, would maintain its angular position in the horizontal plane XY indefinitely, were it. not for the frictional torque caused by its bearings and the drag of the stylus 13. This torque appears about the vertical axis Z and it is a property of gyroscopes,

when subjected to such a torque, to precess or rotate about the axis X. In order to prevent the gyro from precessing a torque opposing the frictional one is applied about the vertical axis Z.

This is done as follows: Attached to the X axis gimbal shaft is an aluminum vane 18 which will rotate in the and a photoelectric .cell 20. The vane 18 is placed so that nolight .falls upon the cell. 20 when the gyroscope wheel 17 is in the X2 plane as said vane 18 cuts off the light beam to the cell 20, and a relay 21 which is operated by the photoelectric cell 20, is de-energized. In the de-energized position relay 21 operates an antiprecession motor 22 allowing it to exert the desired opposing torque to the Z axis shaft 51 through a gear train 23. Actually, this torque is greater than the frictional one, and the gyroscope 17 begins to precess in the opposite direction. As soon as it does however, the vane rotates out of the path of the light, the relay 21 is energized to reverse the armature connection to 'the motor 22, and the motor tends to aid the frictional torque. In this way the gyro wheel 17 hunts about its properxposition in the XZ plane, and renders the system permanently stabilized in the horizontal or XY plane; or in any other single plane depending on how the system is mounted.

This apparatus, when installed in an airplane, provides an automatically recorded polar plot of the antennapattern while the pilot flies in a skid turn as indicated in Fig. 4, of as small a radius as. possible without banking. When the transmitting source 70 is very. distant, say 50 miles, this is equivalent to the ideal method which would suspend the airplane 71 in air and rotate it in the horizontal plane about a fixed vertical With the exception of the transmitting source, the entire system is mounted in the airplane.

Adjustment of the gain of receiver 3 as indicated at 4, of the input to peaking device 6 as indicated at 5 and of the reference potential 8 as indicated at 9, permits the apparatus to be correctly adjusted for optimum results. To further enhance the operating performance of the system it is desired to have stylus 13 be close to the edge of the polar graph paper 15 when the maximum signal is being received during the recording of a particular antenna pattern 25. At the same time it is desired to have. the receiver 3 operate at an optimum point of sensitivity, i. e., at a point of high gain but sufficiently below saturation. It is also desired to have the values of the mean voltages present at output of peaking device 6 and reference source 8 to be of an order to minimize effect of noise and also be considerably larger than the voltage supplied by D. C. generator 12.

In its preferred embodiment, the attenuator 2 is somewhat similar to a section of wave guide in construction. It comprises an outer member 52, an inner, member 53 which. telescopically engages the outer member and a pair of magnetic coupling loops 54 and 55. Loop 54 represents the input loop to which the antenna 1 is connected, and loop 55 represents the output loop to which the receiver 3 is connected. The attenuatordilfers, however, from the normal wave guide in that its cross-sectional dimensions are smaller, than that which would pass a wave 0f the fundamental propagation mode at the frequency at which the attenuator is to be used; That is, the cross-sectional dimensions are smaller than that for cut-afi- It is well known to the art that a device of this nature will produce an attenuation to the energy coupled in that varies as a logarithmic function of its length. That is, the logarithm of this attenuation will be directly proportional to the distance along the attenuator the energy must travel between input and output coupling loops 54 and 55. The couplings may be fixed eachto a section of an attenuator of two sections, one section fitting, inside the other and sliding back and forth trombone-fashion to provide the variation in distance along the guide between couplings. Or, one coupling may be made free to'move to and away from the other coupling while the total length of attenuator cavity remains fixed. This latter method is easily adaptable "to use in an attenuator of circular construction.

It will be seen that since attenuator 2 is connected directly to h recor ing stylus 13, the dimensions and the interpretation of the radiation pattern 25 will depend on the nature of the action of attenuator .2. If attenuator 2 has an attenuation that, yarieslogarithmically ain'egeee with *ais sa'eeasrt, its resulting-antenna?pattern 'coi1ld "be interpretedasdecibelsalong'a'linear radial scale. If attenuator 2 had an attenuation that is linear "with displacement, the resulting antennapattern could be interpreted as'volts along a linear radial scale. The range of attenuation which can be provided by attenuator 2 of the wave guide type depends on its attenuation per unit "length and is total length. Neither of thesetwo variables has an arbitrary limit. When-an attenuator has been the gear ratio. Other attenuators than the wave guide type may be used, many ofwhich' are known to the art. In general, the peakingdevice 6 is thedevice preferred m'rouow receiver 3 when a pulse type transmission is 'bfein'g" received. However, if the noise'peaks are" quite highand numerous, it would be advantageousinfcertain eases'to obtain a voltage that is nearer to the effective value'of the pulses rather than the peak value. If continuous wave transmission and reception is used, no pulse peaking device is required. I

Fig. 2 shows provision made for-these various types "of transmission and reception. In this way, substantially any conventional type of receiver can-be used. In a manner similar to that of Fig. l, the signal is brought from antenna 1, through attenuator 2 to receiver 3. From there" it"is applied to a selector switch 26 which has three positions A, B, and C. Position 'Ainclude's the 'p'ulse stretcher-6b and peak voltage -detector '6q,"and-may be used for normal pulse reception; position B includes in its'path a pulse amplifier 72 which is similartothe pulse stretcher 6b except it produces anout'pnt' signal proportional to the RMS valueof the inputfia'nd maybe used for pulse reception when there is considerable noise -i'nterference; position C may be used whenthe transmission is a continuous wave.

' hen recording an antenna pattern 25 it is'desir'ed to have a ready reference means; to identify the pattern absolutely with the angular orientation'of the antenna '1. :This maybe accomplished by a solenoid, such as that indicated at 24, arranged so that when energized momentarily by pushing a button, it causes a second stylus 57 t'o' give a quick radial movement, superimposing-a small marker pip 58 on the circle normally traced by the second stylus 57. This stylus 57 is located at a'known angle on the polar graph paper with'reference to the main stylus 13. This can be done at any time and in the case of an airplane installation may originate ona signal from the pilot at a given compass bearing or,'for' example, when the airplane is on a heading towards the transmitter. This also assists in the comparison'of more than-one pattern.

When the apparatus of Fig. l is installed in an airplane "it is possible to obtain a radiation'pattern in-thevertical plane of antenna 1 as well as in the horizontal plane. For this, the apparatus is installed, for example, in a horizontal position in line withthe wings and perpendicular to the fuselage. Then the airplane is'flown in a circle in a vertical bank so that the apparatus is now mounted ;which drives the variable attenuator 2. The peaking device 6 includes a pulse inverter 29,'video amplifier 30,

; a pulse'stretcher circuit 6b and a peak voltage detector 6a. The pulse, stretcher circuit 6b includes several vac- .uum tubes 32, 33, 36, 37 connected in cascade. The first zttube 32-isconnected as a cathodelfollowenxanddelivers iwer'tically with the platform'16 again upright. Except a for this change, the action is similar to that for obtaining 'of the balance detector 7.

'its'output to the second tube '33 mascara-Laureate.

The second tube 33 isfconnected as a diode with cathode load consisting of a resistor'35 and shunting capacitor 34. Capacitor 34 charges upon conduction of tube 32" and together with the resistor'35 has a time constant that'in general is considerably larger than the duration ofitlie pulse. The signal voltage developed across the cathode load of tube 33 is'connected'to the grid of the third fourth tube 37 is fed to a peak voltage. detector 6a;

in this embodiment produces steady-output signal that is very closely proportional to the peak valueof a signal impressed on its input terminals. Theout'putsignal' from the peak voltage detector 6a" is fed in series with the output from a suitable anti-hunt generator: 12 to one inpt'it The balance detector 7 includes a pair of vacuum 41 and 42 connected in cascade, and so associated-with the servo actuator 10 as to control thedirection of rotation of shaft 11 according to the sense of output signal from the peak voltage detector 6 relative to a' fixed reference potential. In particular, the output signal from peak voltage detector 6a is preferably positive and is. applied in series through the anti-hunt generator'12 to the. grid of tube 41. At this point.the signal is comparedwiththe fixed reference potential 8 which is applied to the cathode of tube 41 via potentiometer 9. In the-balanced condition, potentiometer 9 is adjusted sothat' both tubes 41 and 42 are conducting a certain amount of current. "f when, however, the output from the peak voltage detectorincreases, the conduction current in tube 42 decreases. As it does so, it applies a differential bias to a pair "of tubes in the servo actuator 10, which in turn causes a motor to rotate shaft 11 in one direction. Under "the circumstances where the' output signal from the peak voltage detector 6a decreases, the conduction current in the'tub'e' 42 increases. In this case the differentialbias applied to the tubes in the servo actuator 10 causes the motor to rotate shaft 11 in the opposite direction. In the servo actuator 10, the pair of tubes-which control the rotation of shaft 11 are indicated at"4 3 and 44. The driving motor for operation of shaft 11 is indicated at 46 comprising a split field winding 45 connecting the plates of tubes 43 and 44 in push-pull relationship-""In the balanced condition of the balanced detector'fi-both tubes conduct equal currents and motor is heldsta ti'dns tor 12, the rotor of which is ganged to, and rotatd'with shaft 11, produces a signal voltage which 'opposes the output of the peak voltage detector 6a so as to reduce hunting by motor 46. i To improve the performance of motor 46 the armature current applied thereto should be stabilized, such asby means of a lamp bank as indicated at 47.

In the embodiment of Fig. 3 the" output from thereceiver 3 is applied to a selector switch 27 which has'two positions of contact D and E; position -D is for-pulse transmission and position E is for continuous wavetran'smission. In position E the output of the receiver3"is applied directly to the input of the peak voltage-detector 6a. In position D the output of the receiver is applied to a second selector switch 28 which also has tWopos'itions of contact F and G; position F for a positiye'pulse output from the receiver and position G for anegative 1. In an apparatus,

, entation varying means.

pulse output from thereceiven, In position F the signal is applied to avideo amplifier 3.0 through an inverter, and position G the signal is applied directly to the video amplifier 30. From the video amplifier 30 the signal is applied to a third selector switch 31 which also has two positions of contact H and I; position H is provided for IRMS operation as hereinabove described, and position 11's provided for peak signal operation. In position H the output from the video amplifieris applied'to the grid of tube 36 inthe pulse stretcher circuit 6b, and in position I lhcoutput from the video amplifier is applied to the grid Of the first tube 32 inrthe pulse stretcher circuit.

Although I have shown and described only a limited and spocific emhodiment of the present invention it is to be understood that I am fully aware of the many modifications possible, thereof. Reference therefore will be had to the appended claims fora definition of the limits ,portional to the output signal from said series connection,

asource ofreference potential,,means comparing said reference potential with the: output signal from said detectiug means, control means responsive to said last :named meaus for sooperating said attenuating means as to maintain ,abalance betweentsaidreference potential and said output signal from said signal detecting means,

,an indicating andlrecording device arranged so as to indicate and record the action of said attenuating means, and means synchronizing said recording device with said ori- -2, In combination, an energy receiving means including series connection with said receiving means, said at- -,tenuating meansincluding vat least one moving element 1 for controlling the attenuation thereof,rsignal detecting means arranged to produce a steady output signal proportional to the output signal from said series conneca tion, a source of reference potential, means comparing said reference potential with the output signal from said detecting means, control means responsive to said last named means for so operating the moving element in said attenuating means as to maintain a balance between said reference potential and said output signal from said signal detecting means, a recording device including a recording medium and a recording element, means for varying the orientation of said antenna relative to a remote source of radiated energy, means synchronizing said recording medium with said variable antenna orientation, and means synchronizing said recording ele- ,ment with the moving elementin said attenuating means.

3. In, an apparatus for indicating and recording the radiation pattern of an energy receiving means, an energy attenuating means arranged in a series connection with said receiving means, said attenuating means including, at least one moving element for controlling the attenuation thereof, signal detecting means arranged to produce a steady output signal proportional to the outputtsignal from said series connection, a source of reference potential, comparing means for deriving a voltage proportional to the difference between said reference potentialand theoutput signal from said detecting means,

, control means responsive to the output of said comparing means :for so operating the moving element in said attenuating means as to maintain a balance between said reference potential and said output signal from said signal detecting means, an indicating and recording device including a recording medium and a recording element, means for varying the orientation of said energy receiying means relative to a remote source of radiated nn antenna, an energy attenuating means arranged in a ene y, e s synchronizin a d re di s m di m with said varying orientation of said receiving means, and

moving element in said attenuating means.

4. In co rnbinatlon, an energy receivingmeans, an energy, attenuating means arranged n a series connection means synchronizing saidrecordingelement with the with said receiving means, said attenuating means includ ing at least onemoving element for controlling the attenuationqthereof, signal detecting means arranged to produce a steady, output signalproportional to the output signal of said, series connection, a source of reference potential, (means comparing said reference potential with the output signal from said detecting means, control means responsive to said last namedmeans'for so operating the: moving elementfof said attenuating means as to maintain a balance between said reference potential and said output signal from said: signal detecting means, means for varying the orientation of said energy receiving means, an; indicating and recording device including a recording medium, and a recording element, means synchronizingsaidrecording medium with said orientation varying means, and means synchronizing said recording element with the. rnoving element of said attenuating means.

5. In a mobile'installation moving in a circular path having a center located'a predetermined distance from a source of radiated-energy, energy receiving means, energy attenuating means connected to said energyreceiving means, signal detecting means connected to said 7 energy attenuating means for providing a control signal having a magnitudeproportional to the output of said energy attenuating means, a source of signals providing a reference signal, having a selected magnitude, means connected to said'signal detecting means and. to said source of "signals for deriving a difierence signal in dc pendency on the difference in the magnitudes between said referencesignal and said control signal, means connectcd between said energy attenuating means and said last mentioned means and responsive to theditfereuce signal for operating said energy: attenuating: means so as 'to :maintain'th'e difference in magnitudes between said control signal and said reference signal at a constant value, a recording device including a recording medium, a recording element, means for connecting said recording element tothe energy attenuating means and for positioning, said recording element in contact with the recording medium, and means connected to said recording device for maintaining the, angle between a selected line in the recording device and a reference line in a predetermined plane substantially: constant as said mobile installation moves in said circular path.

6. In a. mobile installation moving in a circular path havingacenter located a predetermined distance from a source of radiant energy, energy receiving means, energy attenuating, means connected to ,said energy receiving means, signal detecting means connected to said energy attenuatingfmeansfor providing a control signal having a magnitude proportional to the output of said energy attenuating means, a source of signals providing a reference signal having a selected magnitude, means connected to said signal detecting means and to said source of signals for, comparing the magnitudes of said reference signal and said control signal, means connected between said energy attenuating means and said last mentioned means and responsive to the output of said last mentioned means for operating said energy attenuating means so as to maintain the ratio of magnitudes of said control signal and said reference signal constant, a platform having arecording medium-positioned thereon, a recording element, means for connecting said recording element to the energy attenuating means and for positioning said recordingelement in contact withthe recording medium, .a gyroscope, means for connecting said gyroscope to said platform,, and means connected to said gyroscope for maintaining ,theangle'bctween a selected line through the gyroscope and a reference line in a predetermined plane substantially constant while said gyroscope is in rotation.

7. In a mobile installation moving in a circular path having a center located a predetermined distance from a first radiant energy source, energy receiving means, energy attenuating means connected to said energy receiving means, signal detecting means connected to said energy attenuating means for providing a control signal having a magnitude proportional to the output of said energy attenuating means, a source of signals providing a reference signal having a selected magnitude, means connected to said signal detecting means and to said source of signals for deriving a difference signal in dependency on the dilference in the magnitudes between said reference signal and said control signal, means connected between said energy attenuating means and said last mentioned means and responsive to said difference signal for operating said energy attenuating means so as to maintain the difference in magnitudes between said control signal and said reference signal at a constant value, a platform having a recording medium positioned thereon, a recording element, means for connecting said recording element to the energy attenuating means and for positioning said recording element in contact with the recording medium, a gyroscope including a shaft, control means connected to said shaft, said control means having a first operating condition in which a desired torque is applied to said shaft with a component opposing the frictional torque on said shaft and a second operating condition in which a desired torque is applied to the shaft with a component aiding said frictional torque, a second radiant energy source, a photo cell, means connected to said shaft and positioned between said second radiant energy source and said photo cell for permitting energy from said second radiant energy source to impinge on said photo cell only when the angle between a selected line through said shaft and a reference line in a predetermined plane exceeds a desired value, and means connected between said photo cell and said control means for placing the control means in said first operating condition when energy from said second radiant source fails to impinge on said photo cell and in said second operating condition when energy from said second radiant source impinges on said photo cell.

References Cited in the file of this patent UNITED STATES PATENTS 2,519,418 Urick Aug. 22, 1950 2,602,924 Schmitt et al. July 8, 1952 2,647,813 Watson Aug. 4, 1953 2,701,876 Mottley et al. Feb. 8, 1955 

